The central goal of this study is to identify factors associated with children's differential response to an empirically validated treatment designed to reduce serious youth antisocial behavior and delivered in community-based settings. Specifically, this longitudinal study will examine a transactional model of treatment response and non-response that focuses on the interaction of child (biological), family, environment and treatment adherence variables that are likely to influence treatment outcome in community mental health care settings. A total of 185 families receiving an evidence-based treatment (Multisystemic therapy; MST) in community settings will be assessed 5 times (pre treatment, twice midtreatment, post treatment, follow-up) using a multi source, multi method assessment battery. Components of the transactional model guiding this investigation will be tested by linking child factors that have been identified in basic science (child emotional responsiveness, indexed by cortisol levels) and treatment outcome research (parent psychopathology, therapist adherence, parenting practices, environmental stressors) as potential mediators and moderators of treatment outcome among antisocial youth. The general aims of this single group longitudinal study are to: Aim 1: Examine individual, family, and environmental factors that may contribute directly and/or indirectly to treatment adherence to evidence-based treatment procedures and to treatment outcome. Aim 2: Evaluate the role of therapist adherence in predicting changes in caregiver behavior; and determine whether interactions among pretreatment individual and family characteristics and environmental stressors during treatment moderate the association between adherence and changes in family practices. Aim 3: Determine whether changes in parenting practices during treatment mediate the relationship between therapist adherence and changes in youth antisocial behavior. Aim 4: Examine basic science questions regarding treatment related changes in youth emotional reactivity in the context of evidence-based treatment.